Moisture control system including control of pulp flow to a paper machine headbox in response to moisture measurement

ABSTRACT

ON A PAPER MACHINE WHEREIN THE DRIERS ARE OPERATING AT FULL CAPACITY, THE MOISTURE CONTENT OF THE PAPER WEB COMING OFF OF THE MACHINE IS MEASURED. IF THE MOISTURE CONTENT SO MEASURED DEVIATES FROM THE DESIRED MOISTURE CONTENT, CORRECTIVE ACTION IS TAKEN THROUGH READJUSTMENT OF THE BONE DRY BASIS WEIGHT. FLOW RATE OF STOCK TO A PAPER MACHINE HEADBOX IS INCREASED WHEN THE MOISTURE CONTENT IS LOWER THAN THE DESIRED MOISTURE CONTENT AND THE FLOW RATE OF STOCK TO THE HEADBOX IS DECREASED WHEN THE MOISTURE CONTENT IS HIGHER THAN THE DESIRED MOISTURE CONTENT.

March 14, 1972 J. M. FuTcH. JR

MDlSfUHIu' CONTROL SYSTEM INCLUDING CONTROL OF PULP FLOW TO A PAPER MACHINE HEADBOX IN RESPONSE TO MOISTURE MEASUREMENT Filed May 15, 1969 3 Sheets-Sheet 1 mm jOmkzOo Km IOEPZOO R. my H C m F M s E M A J AGENT March 14, 1972 J. M. FUTCH. JR 3,5

CONTROL SYSTEM INCLUDING CONTROL OF PULP FLOW TO A PAPER MACHINE HEADBOX IN RESPONSE TO MOISTURE MEASUREMENT 3 Sheets-Sheet 2 Filed May 15, 1969 z. 4 "6.2 535 mw 5 23w Emma 993 I l I mm I I I m m $2 533 mv 3 wKDFQOE om wmc KMJJOWEIZOU INVENTOR JAMES M. FUTCH. Jr.

AGENT March 14, 1972 J. F H, JR 3,649,444

MOlS'lUlH-L UUN'I'RDE SYSTEM INCLUDING CONTROL OF PULP FLOW TO A I'APER MACHTNE HEADBOX IN RESPONSE TO MOISTURE MEASUREMENT Filed May 15, L869 3 Sheets-Sheet Z READ OUTPUT OF MoISTURE GAGE (m CALCULATE MEAN MOISTURE CONTENT OF HE WEB (E1) 64 READ DESIRED MEAN MOISTURE CONTENT (ri es CALCULATE MOISTURE ERRoR (m m =fi fi CALCULATE REQ. CHANGE IN TS F VALVE POSITION(AV) Av=m [K(s)] I ACTUATE VALVE 70 WAIT FOR CONTACT CLOSURE FROM MOISTURE GAUGE INVENTOR JAMES M. FUTCH Jr FIG. 3 BY I AGENT United States Patent 3,649,444 MOISTURE CONTROL SYSTEM INCLUDING CON- TROL OF PULP FLOW TO A PAPER MACHINE HEADBOX IN RESPONSE TO MOISTURE MEASUREMENT James M. Futch, Jr., Yonges Island, S.C., assignor to Westvaco Corporation, New York, N.Y. Filed May 15, 1969, Ser. No. 824,857 Int. Cl. D21f 1/06 U.S. Cl. 162-198 13 Claims ABSTRACT OF THE DISCLOSURE On a paper machine wherein the driers are Operating at full capacity, the moisture content of the paper web coming off of the machine is measured. If the moisture content so measured deviates from the desired moisture content, corrective action is taken through readjustment of the bone dry basis weight.

Flow rate of stock to a paper machine headbox is increased when the moisture content is lower than the desired moisture content and the flow rate of stock to the headbox is decreased when the moisture content is higher than the desired moisture content.

BACKGROUND OF THE INVENTION Field to which the invention pertains In the manufacture of paper the fibers which will ultimately be formed into a paper sheet or Web are supplied to the paper machine complex in the form of an aqueous suspension, the density or consistency of which is typically 34%, i.e. on a weight basis, 3-4% of the aqueous suspension is fiber. Within the paper industry, this suspension is referred to as the thick stock or thick stock flow.

Within the paper machine complex, the consistency of the thick stock flow is reduced by the addition of socalled White water. The diluted thick stock flow, which typically is approximately 99.5% water or a consistency of 0.5%, is pumped to a head-box which discharges the dilute suspension on to a moving, foraminous belt, i.e. the fourdrinier wire. From this point on, all operations and processes on the paper machine have as their objective either the removal of water from the sheet in order to adjust the moisture content thereof or the conditioning of the sheet as to its surface appearance. The invention described herein concerned with the former objective, i.e. moisture control.

After the pulp suspension has been deposited on the wire, a major portion of the water is removed by a combination of table rolls or foils as well as certain vacuum equipment which are located under the wire. The water thus removed is delivered to a holding tank referred to as the wire pit. This water is the so-called white water referred to earlier. Generally, the moisture content of the sheet leaving the wire will be on the order of 80%.

From the wire, the sheet will pass to and through a press section wherein it passes between a series of large rollers, additional water being removed therein through mechanical expression. As the sheet leaves the press section, having a moisture content of approximately 60%, it enters the so-called drier section. In this section the sheet passes between a series of heated rolls wherein steam is generally used as the heating agent, i.e. steam is passed through the inside of the rolls thereby heating the surface of the roll. When the paper sheet or web emerges fiom the drier section it is in its final form and is wound onto a reel.

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The moisture content of the web leaving the drier section is an important quality of the paper since it has a profound affect on many other proporties of the sheet, e.g. its strength. For this reason, the moisture content of the sheet is measured as the sheet exists from the drier section.

If the actual moisture content deviates from the desired moisture content, the usual practice is to alter the conditions in the dried section by, for example, varying the steam flow to the driers. However, a problem is presented when conditions are such that the driers are operating at their maximum capacity since, under such conditions, the driers are not available as a control mechanism for the moisture content of the sheet.

When a paper machine is operating under such a condition, the machine is said to be drier limited.

The method by which one controls the moisture content of a sheet produced on a drier limited machine is the field to which my invention pertains.

PRIOR ART The prior art has recognized that on a non-drier limited paper machines, control of the sheet properties could be effected by using the thick stock flow rate to control basis Weight and heat input to the drier section to control moisture. At least two reasons indicate that this is the obvious mehod of controlling basis weight and moisture on a non-drier limited paper machine. First, assuming that consistency remains constant, control of basis weight by manipulation of the thick stock flow rate essentially effects a material balance. Second, since the driers are the last moisture removal operation and since they are located just ahead of the moisture measuring point, there is a minimum time lag between a change in drier operation and the time when such change is detected by the moisture measuring equipment.

When the prior art was faced with the problem of controlling basis weight and moisture on a drier limited paper machine, the obvious approach was to follow the material balance principal used on non-drier limited machines and control basis weight by manipulating the thick stock flow rate. However, since the paper machine was drier limited, manipulation of the heat input to the driers was no longer available as a mechanism for controlling moisture. Thus, the prior art used wire speed to control moisture, i.e. if the moisture content of the sheet increased above the desired value, the machine speed was decreased thereby providing a greater residence time of the sheet within the drier section, the result of which would be a decrease in the moisture content. Alternatively, a decrease in moisture content would cause an increase in machine speed which would reduce the residence time of the sheet within the drier section and thereby increase the moisture content.

This approach to moisture control on a drier limited machine is not, however, without difficulties. Specifically, these difiiculties result from the interaction of basis weight and moisture which attends the use of this control scheme. For example, if in response to an increase in moisture the wire speed is decreased as mentioned above and if this is the only control action taken, one obtains no significant change in moisture. This absence of a change in moisture results from the fact that if the wire speed is decreased and the thick stock flow rate is held constant, there will be an increase in basis weight. As such, although the web will have a greater residence time in the driers, there will be no substantial change in moisture because of the increased basis weight. This interaction is manifested by an open loop gain, between moisture and wire speed, of essentially zero.

The prior art, in order to utilize Wire speed as a manipulated variable for the control of moisture, has thus been forced to utilize an interacting control system wherein a change in moisture will cause a change in wire speed and the change in wire speed may cause a change in basis weight which may cause a change in thick stock flow rate. As will hereinafter be pointed out, through the use of my method, such interaction is substantially eliminated.

SUMMARY OF THE INVENTION On a drier limited paper machine, rather than controlling moisture by manipulating wire speed, moisture is controlled by manipulating the bone dry basis weight, e.g. by manipulating the thick stock tflovv rate. If control of basis weight is required, e.g. during a grade change, such control would be realized by manipulating Wire speed.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representation of a drier limited paper machine and the preferred embodiment of my method.

FIG. 2 is another embodiment of my method.

FIG. 3 is a flow chart representation of my method for a digital implementation thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Stated succinctly, I have discovered a superior method of controlling the moisture content of a paper sheet produced on a drier limited paper machine, viz. by manipulating the fiber content of the sheet. In the paper industry the fiber content of the sheet is referred to as the bone dry basis weight (BDBW) Where as the term basis weight indicates the Weight of the sheet including both the fiber content (BDBW) and the moisture content thereof. Hereinafter, the application of my method will be described as well as the system dynamics which seem to explain the superiority of my method as compared to the prior art methods of controlling moisture.

While the essence of my invention is the control of the moisture content of a paper sheet by effecting small changes in the BDBW thereof, it will be appreciated that BDBW may not be manipulated directly, i.e. some other variable which may be manipulated directly must be utilized to effect changes in the BDBW, for example the thick stock flow rate or consistency. Therefore, the following description of my invention wlil relate to the control of the moisture content of the sheet through the manipulation of a variable which is available for direct control,

e.g. the thick stock flow rate. However, it will be understood that the objective of manipulating a variable such as the thick stock flow rate is to cause a change in the BDBW of the sheet by changing the consistency of the stock within the headbox and thereby control moisture. In the preferred embodiment of my method, as hereinafter described, the thick stock flow rate is used as the available manipulated variable by which changes in the BDBW are achieved in order to control the moisture content of the paper web or sheet.

FIG. 1, which represents the preferred embodiment of my invention, shows in block diagram form all the elements of what was hereinbefore referred to as the paper machine complex, i.e. all the apparatus elements within the box -9. Thus, referring to FIG. 1 and as pointed out earlier, thick stock is supplied to the line and passes through the thick stock flow control valve 11 to the line 12. The thick stock in line 12 is mixed with white water from the wire pit 13 and is pumped by the fan pump 14 through the line 15 to the headbox 16. The dilute stock passes from the headbox 16 onto the moving, fourdrinier wire 17. A substantial portion of the water which comprises the dilute slurry is drained through the wire into the wire pit 13. The remainder passes off the wire as at 22 and subsequently under oes a number of processing operations, e.g. the formed Web may be passed through press rolls such as 19. The web, as at 24, contains substantially less Water. After the press rolls, the web would pass through a dryer section 20. All of these elements, as well as their purpose and function, are well known to those skilled in the papermaking art. Further, for the purposes of describing my invention, it will hereinafter be understood that the paper machine complex as shown is operating as and is representative of a drier limited paper machine. Still further, for purposes of explanation only, certain operating conditions will be assumer, which conditions are, in fact, typical of those which one would find on a so-called kraft linerboard machine, i.e. a paper machine manufacturing paper for use in corrugated paperboard.

It will be assumed that hte desired moisture content of the web 40, when it leaves the drier section 20, is 5% 11% and the desired basis weight at that point is 42 pounds, i.e. the web weights 42 pounds per 1,000 square feet. The desired machine speed, i.e. the linear speed of the Web will be assumed to be 1,800 feet per minute.

In FIG. 1, as the paper web 40 leaves the drier section 20, moisture gage 30 traverses the width of the moving Web and continuously measures the moisture content thereof. Moisture gage of this type are well known and commercially available. The output of the moisture gage 30 is fed to scan average calculator 32. The purpose of this device is to continuously receive the instantaneous output of the moisture gage 30 and, at the end of each scan, i.e. each time the moisture gage makes one traverse of the web, produce an output signal which is equal to, in this embodiment, the statistical mean of the continuous moisture signal. The scan average calculator is a standard piece of hardware which is commercially available as part of the traversing moisture gage 30.

From an information point of view, the output of the scan average calculator is discontinuous since a new scan average is only available at the end of each scan. Thus, in the analog embodiment of my invention shown in FIG. 1, the analog control loop operates discontinuously.

The output 42 of the scan average calculator 32 is compared with the desired scan average 33 by summing junction 41 the output of which is an error signal 43 which serves as an input to the analog controller 45. The analog controller 45 may be any standard and commercially available controller which provides an integral control action, i.e. the output 47 is equal to the input error signal 43 multiplied by a constant, K, which is adjustable. The output 47 of the controller 45 is a signal to the thick stock flow control valve 11. The thick stock flow control valve may be actuated by any conventional means, e.g. pneumatic, hydraulic or electrical means, the only requirement being that the actuation means are compatable with the controller output 47 Which is, in this embodiment, discontinuous.

As previously pointed out, prior to my discovery of the superiority of controlling moisture through the manipulation of thick stock flow rate, the prior art had used thick stock flow rate as the manipulated variable to control basis weight. Of course, if thick stock flow rate is used as the manipulated variable to control moisture according to my discovery, it will not be available for manipulation to control basis weight. Thus, the question is presented as to how, if at all, the basis weight may be controlled.

During essentially steady state conditions, i.e. when a grade change is not occurring, I have found that basis weight control is generally not required. Probably, this arises from the fact that, as will be pointed out, within the range of normal operating conditions moisture is ten times more sensitive to changes in thick stock flow rate than basis weight, and therefore if the moisture-thick stock flow rate control loop maintains a constant moisture, the basis weight, perforce will, be essentially constant assuming that other parameters remain constant, e.g. speed. Moreover, because of the high sensitivity of moisture to changes in thick stock flow rate, only small changes in thick stock flow rate will be required to control moisture and, in general, these changes will be so small that either they will not cause a change in basis weight or if they do, the changes thus caused will be within the allowable limits by which the basis weight may deviate from the desired value. Alternatively, if external conditions should cause a change in thick stock flow rate, this change will be detected as a moisture change before it is detected as a basis Weight change. Thus, control action will be taken because of the moisture deviation before a substantial basis weight deviation occurs.

However, should One desire to have available regulatory means for basis weight control, I have further perceived that one may achieve such regulation by using speed as the manipulated variable to control basis weight. Referring once again to FIG. 1, the means required to effect this result are shown.

In FIG. 1 a basis weight gage 70 traverses the moving web in the same manner as heretofore described with reference to the moisture gage 30. Although in FIG. 1 the moisture gage 30 and the basis weight gage 70 are shown as being aligned in the machine direction, they could equally well be positioned in tandem in the cross machine direction.

As the basis weight gage 70 traverses the web, the instantaneous output signal 69 is received by a basis weight scan average calculator 71 which performs a function similar to that of the moisture scan average calculator 32, i.e. at the end of each scan the basis weight scan average calculator 71 produces an output signal 72 equal to the means basis weight. The mean basis weight 72 is compared to the desired average basis weight 73 by the summing junction 74. The result of this comparison is an error signal 75 which is the input to the speed controller 76 which is functionally similar to the moisture controller 45 hereinbefore described. The output 77 of the speed controller 76 controls the speed of drive motor 78 and thereby, through the belt 80, controls the wire speed. Thus, ancillary to the practice of my invention per se and in order to facilitate a grade change, basis weight may be controlled by manipulating wire speed.

Another analog implementation of my method is shown in FIG. 2. For clarity, the apparatus which comprises the paper machine complex and was shown in FIG. 1 enclosed by the dashed enclosure has been omitted and only the enclosure 9 is shown in FIG. 2. It is to be understood that the signals 31 and 69, the control signal 77 and the process input all correspond to the similarly numbered signals and inputs of FIG. 1.

In the embodiment shown in FIG. 2 the thick stock flow rate is controlled by a cascade control loop wherein the moisture control loop is cascaded onto a local flow control loop. A moisture control signal 47 is obtained as previously described, however, instead of actuating the thick stock flow control valve 11 directly, the moisture control signal 47 is fed to a cascade station 100 which converts the moisture control signal 47 into a thick stock flow rate set point 101. The thick stock flow rate set point 101 is then compared to a signal 82 by comparator 102. Signal 82, which represents the actual thick stock flow rate, is obtained by the flow transducer 81. Both the flow transducer 81 and the cascade station 102 are commercially available pieces of hardware. Of course, the cascade station 100 would be provided with a memory circuit for retaining the moisture control signal 47 in the cases Where the moisture control signal 47 is discontinuous.

As a result of the comparison, by comparator 102, between the thick stock flow set point signal 101 and the signal 82 representing the actual thick stock flow, an error signal 103 is obtained which is the input signal to controller 104. Controller 104 may be standard, commercially available one, two or three mode analog controller depending upon the control action desired. The output 105 of controller 104 actuates the thick stock flow control valve 11 thereby controlling the thick stock flow.

Of course, this cascade method of controlling moisture has the benefit of detecting and correcting changes in the thick stock flow rate before such changes cause an upset in moisture. The basis Weight control loop in FIG. 2 opcrates as described above with reference to FIG. 1.

Although an analog embodiment of my method has hereinbefore been described with reference to FIG. 1, the preferred implementation of my method would employ digital control means. A direct digital control implementation of my method is described in FIG. 3 which is essentially a flow sheet of the steps which a digital control computer might follow to realize my method of control.

Referring to FIG. 3, as the moisture measuring gage traverses the web its output is periodically read 60 by the computer. When the gage reaches the end of a traverse, a contact is physically closed by the gage and a signal is sent to the computer indicating the end of a scan. When the signal from this contact closure is received in the computer, there is a priority interrupt and the computer calculates 62 the mean of all the signals read during the traverse. The result of this calculation, i.e. the actual mean moisture content of the paper, is compared with the desired average moisture content as read from the computer memory 64 and thus a moisture error signal is calculated 66. This moisture error signal is then used to calculate 68 the required change in the position of the thick stock flow control valve. This change in valve position, delta V, is calculated by multiplying the moisture error signal by K(s) where K(s) represents a control algorithm. While I have used a single (integral) action velocity algorithm in the digital implementation of my new method, numerous other algorithm are known to those skilled in the control art and may be selected according to the requirements of the particular digital computer which is available and the type of control response desired. After the thick stock flow control valve has been actuated 70, the computer returns to other calculations until a new priority interrupt occurs, i.e. a contact closure from the moisture gage.

Although there are a large number of apparatus arrangements available for implementing my method, e.g. either analog or digital with different versions of each, the fundamental basis of my method relates more to the papermaking process than the apparatus used to implement the method. When I first discovered the method of controlling moisture by manipulating the thick stock flow rate and thus the weight per unit area of the sheet, the explanation of why this method was superior was unknown. However, subsequent investigation appears to have uncovered the system dynamics which explain the superiority of this method and will now be described.

After my discovery of the method described herein, investigations were conducted with the objective of determining the open loop gain (open loop gain:

cv -ct /(AMV) where CV the desired value of the controlled variable, e.g. moisture, CV=the actual or measured value of the controlled variable and AMV=the change required in the manipulated variable to make (CV =CV between basis weight vs. thick stock flow rate and moisture vs. thick stock fiow rate. For a machine of the type previously described, i.e. a kraft linerboard machine operating under dried limited conditions, it was determined that the open loop gain between basis weight and thick stock flow rate was 2.3 10 (lb/1000 ft. )/g.p.m. while the open loop gain between moisture and thick stock flow rate was 0.26 10- (moisture fraction)/g.p.m. wherein the term moisture fraction means the moisture of the sheet eX- pressed in decimal form rather than as a percentage, i.e. rather than speaking of a 5% moisture, the moisture fraction would be 0.05. At first encounter, the obvious implication to be drawn from these two values of open loop gain would be that basis weight is more sensitive to changes in thick stock flow than moisture since the absolute value of the open loop gain between basis weight and thick stock flow rate is on the order of one hundred times greater than the absolute value of the open loop gain between moisture and thick stock flow rate. This obvious implication would seem to support the prior art approach of using thick stock flow rate to control basis Weight rather than moisture. However, since this obvious implication did not correspond to the superior control which was realized by utilizing my method, viz. controlling moisture by manipulating thick stock flow rate, further consideration was given to these two values of open loop gain whereupon it was perceived tht one cannot view the relationship between these two parameters in absolute value. That is to say, in addition to considering the value of these two open loop gains, one must also consider the constraints under which the paper machine must operate, eg the minimum, maximum and desired value of both basis weight and moisture. As such, one may obtain a true picture of the control dynamics of a paper machine by considering not the absolute value of the two open loop gains in question but their value expressed in terms of a percentage change in either basis weight or moisture wherein the percentage is based on the desired basis Weight or moisture.

Thus, consider the paper machine heretofore defined wherein the machine was operating under drier limited conditions and the desired basis Weight and moisture was 42 lbs. and respectively. Now, using the two values of open loop gain heretofore given, viz. 2.3 (lb/1000 ft?) /g.p.m. as the open loop gain between basis weight and thick stock flow rate versus 026x10 (moisture fraction) /g.p.m., as the open loop gain between moisture and thick stock fiow rate, it will be seen that a one hundred g.p.m. change in the thick stock flow rate will cause a basis weight change of 2.3 lbs. and a moisture change of 0.026, assuming both open loop gains remained constant for a 100 g.p.m. change in thick stock flow rate. Now, since the desired basis weight is 42 lbs., a change of 2.3 lbs. represents a change of approximately 5.47%. However, since the desired moisture content is 5% or 0.05 in terms of moisture fraction, a change in moisture fraction of 0.026 represents a change of approximately 52%. Therefore, it will be appreciated that although the absolute value of the open loop gain between basis weight and thick stock flow rate is one hundred times greater than the absolute value of the open loop gain between moisture and thick stock flow rate, a given change in thick stock flow rate will produce, in terms of a percentage of desired values, a change in moisture ten times greater than the corresponding change in basis weight. This significantly greater change in moisture as compared to basis weight appears to represent the explanation of the superiority of my method. That is to say, by eiiecting small changes in the weight per unit area of the sheet, one may obtain significant changes in and therefore control of the moisture content of the sheet.

It will now be appreciated that through the use of my method one can avoid the interaction between basis weight and moisture which, as was previously mentioned, attended the use of prior art control methods. This avoidance of interaction results, once again, from the fact that the changes required in the thick stock flow rate to correct moisture deviations are sufiiciently small that the resulting change in basis weight will be within acceptable limits. For example, assume that the machine previously described was operating under conditions such that the desired moisture and basis Weight were 0.05+0.0l (moisture fraction) and 42 pounds-:1 pound, respectively. Based on these conditions and the open loop gains previously given, if one wished to increase the moisture fraction to its upper limit, i.e. from 0.05 to 0.06 or an increase of 0.01 through a manipulation of the thick stock flow control value, the required increase in the thick stock flow rate would be on the order of 38.6 g.p.m. Such an increase in thick stock flow rate would result in a basis weight increase of approximately 0.885 pound, i.e. the basis weight would change from 42.0 to 42.885 pounds which is within acceptable limits. Thus, within the ranges of moisture and basis Weight heretofore mentioned, the interaction found in the prior art methods would be avoided or kept within acceptable limits. Of course, if such changes in basis weight are thought to be unacceptable, other basis weight regulatory means may be employed, e.g. machine speed as described above.

It has been pointed out that the operation of the analog implementation of my method is inherently discontinuous in the absence of some memory means, since there is no continuous signal available which is representative of the actual average moisture content of the sheet. Since this discontinuity of operation is more or less inherent, one may use it to some advantage by inhibiting control action for an amount of time equivalent to the transport lag between the thick stock flow control valve and the moisture gage.

Alternatively, in the digital implementation of my method, the frequency of the control calculation can be as frequent as an average moisture signal is available notwithstanding the fact that more than one calculation might be made before the effect of a previous calculation is detected by the moisture gage since, in the programming of the computer one may provide for the effect of the last calculation. As an example of such an approach, suppose that the transport lag between the stock flow control valve and the moisture gage is seconds and assume further that the moisture gage completes a traverse of the web every 20 seconds. Obviously, if a straight control calculation were run at the end of every traverse the same correction would be made four times (assuming a constant error signal) and the control system would thus over correct with a resulting overshoot. The expedient by which this undesirable result can be avoided comprises the additional steps of memorizing each error signal before the control calculation is completed and subtracting the error signal so memorized from the very next error signal. Thus, assume that during the nth calculation an error signal of one unit is calculated. This error would be read into the computer memory and the remainder of the nth control calculation would be completed wherein sufficient control action would be taken to reduce the error signal of one unit to zero. Let it further be assumed that during the next control calculation, i.e. control calculation n+1, the calculated error signal is still one unit. By the expedient of subtracting the last error signal no control action would be taken and, indeed, none would be called for since it is evident that the error has not changed and appropriate control action has already been elfected. This procedure would be continued, i.e. the error signal for the nth calculation would continue to be subtracted, for four calculation runs since, at that time, the effect of the control action for the nth calculation should be detected.

While the preferred and other embodiments of my invention have hereinbefore been described, other embodiments of my invention will be apparent to those skilled in the art, which are nevertheless within the scope of my invention.

1 claim:

1. On a drier limited paper machine including a headbox for discharging stock onto a fourdrinier wire and a thick stock flow control valve for controlling the flow of fiber to said headbox, the method of controlling the moisture content of a paper web on said machine which comprises:

(a) measuring the moisture content of said paper web downstream of the fourdrinier wire; and

(b) responsive to the measured moisture content of said paper web, automatically increasing the flow rate of fiber to said headbox when the measured value of the moisture is lower than a desired value and automatically decreasing the flow rate of fiber to said headbox when the measured value of the moisture is higher than a desired value.

2. The method of claim 1 wherein the flow rate of fiber to said headbox is changed by repositioning the thick stock flow control valve.

3. On a drier limited paper machine including a headbox for discharging stock onto a fourdrinier wire and a thick stock fiow control valve for controlling the flow of fiber to said headbox, the method of controlling the moisture content of a paper web on said machine which comprises:

(a) generating a first signal representative of the moisture content of said paper web at a point downstream of the fourdrinier wire;

(b) generating a second signal representative of the desired moisture content of said paper web;

(c) comparing said first and second signals; and

(d) increasing the flow rate of fiber to said headbox when said first signal is less than said second signal and decreasing the flow rate of fiber to said headbox when said first signal is greater than said second signal.

4. The method of claim 3 wherein the flow rate of fiber to said headbox is increased by repositioning the thick stock fiow control valve.

5. The method of claim 3 wherein said first signal is representative of the average moisture content of said paper web and said second signal is representative of the desired average moisture content of said paper web.

6. On a drier limited paper machine including a headboX for discharging stock onto a fourdrinier wire and a thick stock flow control valve for controlling the flow of fiber to said headbox, the method of controlling the moisture the moisture content of a paper web on said machine which comprises:

(a) generating a first signal representative of the moisture content of said paper web at a point downstream of the fourdrinier Wire;

(b) generating a second signal representative of the desired moisture content of said paper web;

(c) comparing said first and second signals to obtain a first error signal;

((1) generating a third signal representative of the thick stock fiow;

(e) generating from said first error signal a fourth signal representative of the desired thick stock flow;

(f) comparing said third and fourth signals; and

(g) repositioning the thick stock flow control valve toward its open position when said fourth signal is greater than said third signal to increase the moisture content of the paper Web and repositioning the thick stock flow control valve towards its closed position when said fourth signal is less than said third signal to decrease the moisture content of the paper web. 7. On a drier limited paper machine including a headbox for discharging stock onto a fourdrinier Wire and a thick stock flow control valve for controlling the flow of fiber to said headbox, the method of controlling the moisture content of a paper web on said machine which comprises:

(a) scanning said paper web, in the cross machine direction and at a point downstream of the fourdrinier wire, to generate a plurality of signals representative of the moisture content of the paper web in the cross machine direction;

(b) combining said plurality of signals to obtain a first signal representative of the average of said plurality of signals;

(c) generating a second signal representative of the desired average moisture content of said paper web;

(d) comparing said first and second signals; and

(e) repositioning the thick stock flow control valve toward its open position when said first signal is less than said second signal and repositioning the thick stock fiow control valve towards its closed position when said first signal is greater than said second signal.

8. The method of claim 7 which further comprises the step of periodically inhibiting the repositioning of the thick stock flow control valve.

9. On a drier limited paper machine including a headbox for discharging stock onto a fourdrinier wire and a thick stock fiow control valve for controlling the flow of fiber to said headbox, the method of controlling the moisture content and basis weight of a paper web on said machine which comprises:

(a) generating a first signal representative of the moisture content of said paper web at a point downstream of the fourdrinier wire;

(b) generating a second signal representative of the desired moisture content of said paper web;

(c) generating a third signal representative of the basis weight of said paper web at a point downstream of the fourdrinier wire;

(d) generating a fourth signal representative of the desired basis weight of said paper web;

(e) comparing said first and second signals;

(f) repositioning the thick stock flow control valve toward its open position when said first signal is less than said second signal and repositioning the thick stock flow control valve towards its closed position when said first signal is greater than said second signal;

(g) comparing said third and fourth signals to obtain a second error signal; and

(h) controlling the speed of said paper machine in response to said second error signal.

10. The method of claim 9 wherein said first signal is representative of the average moisture content of said paper web, and said second signal is representative of the desired average moisture content of said paper web.

11. On a drier limited paper machine including a headbox for discharging stock onto a fourdrinier wire and a thick stock flow control valve for controlling the flow of fiber to said headbox, the method of controlling the moisture content and basis weight of a paper web on said machine which comprises:

(a) generating a first signal representative of the moisture content of said paper web at a point downstream of the fourdrinier wire;

(b) generating a second signal representative of the desired moisture content of said paper web;

(c) comparing said first and second signals to obtain first error signal;

(d) generating a third signal representative of the thick stock flow;

(e) generating from said first error signal a fourth signal representative of the desired thick stock flow;

(f) comparing said third and fourth signals;

(g) repositioning the thick stock flow control valve toward its open position when said fourth signal is greater than said third signal to increase the moisture content of the paper web and repositioning the thick stock flow control valve towards its closed position when said fourth signal is less than said third signal to decrease the moisture content of the paper Web;

(h) generating a fifth signal representative of the basis sired basis Weight of said paper web;

(i) generating a sixth signal representative of the deweight of said paper web;

(j) comparing said fifth and sixth signals to obtain a third error signal; and

(k) controlling the speed of said paper machine in response to said third error signal.

12. The method of claim 10 wherein said first signal is representative of the average moisture content of said paper web and said second signal is representative of the desired average moisture content of said paper web.

13. The method of claim 11 which further comprises the step of periodically inhibiting manipulation of the thick stock flow.

(References on following page) References Cited UNITED STATES PATENTS FOREIGN PATENTS OTHER REFERENCES Dipre: New England Closed Loop Control of B.W.,

Alexander Tappi, v01. 41, No. 11 (November 1963), pp. 178A- Lippke 162-198 5 181A- ggzgi f 'g E@ s. LEON BASHORE, Primary Examiner Taylor 162-253 A. DANDREA, JR, Assistant Examiner U.S. Cl. X.R.

Canada. 10 162-253, 258, 263 

